This could be one of two problems, or possibly both:
1. The blend door actuator is not working. Take off the panel below the glove box on the passenger side floorboard. You are looking for a small flat control a bit smaller than a pack of cigarettes. It is located on the bottom of the dash area, just to the right of the transmission hump. As you turn the switch from heat to cool and back, this actuator should move a shaft coming out of the bottom of the heat/AC box and into the middle of this actuator. You will also hear a motor type noise from it while moving the control. If this is faulty, you may have issues with the control sticking on heat or cool, or possibly not switching at all (as in your case). A failure in this component could also be caused by a bad switch in the controls on the dashboard, but this isn't as likely.
2. The heater core is blocked. There are two hoses going through the firewall on the passenger side of the engine compartment. They are roughly the same side as a garden hose, maybe a little thicker, and go through the firewall adjacent to each other. With the truck warmed up, carefully feel each hose. Are they both similar in temperature? If not, the coolant flow in the heater core is likely partially or completely blocked. Try flushing it with a garden hose while it is disconnected from the remainder of the heating system. If you have a complete blockage, you may not be able to flow any water through the heater core. Be careful not to use too much pressure, as you don't want to risk rupturing the heater core, especially if it is already corroded.
I hope this helps, as I have been through the same issue with my Tahoe. Good luck!
SOURCE: 2001 tahoe front heater not working, rear heater blows warm air
Notorious for plugged heater cores. Also updated plastic restriction couplers mounted at heater core available from dealer. Try back flushing heater core. Run engine till its at operating temperature then carefully hold the inlet and outlet hose breifly with heater fan on high if hoses are nearly the same temp then flow is good , if not then core is plugged.
SOURCE: 2003 chevy tahoe no rear heat
I know the earlier tahoe's have two heater lines running from the engine compartment to the rear under the vechile. Those lines could be clogged or pinched shut. What you do is get two hose locking pliers. Go under your truck, they are two metal lines that turn into rubber and back into metal again farther down the line. First, checked for pinched lines. If none, find where the rubber hoses are, clamp them off, remove from metal lines and drain. Use garden hose to flush fresh water through. When water comes through at high pressure and pretty clear, it should be done. Reconnect hoses, top off radiator fluid and it should be fine.
SOURCE: Rear a/c not blowing cold, front works fine.
you may not have enough refrigerant. a low charge can cause the rear in this dual system to not work for beans. charge with a can of uv and find the leak. fix the leak. evac the system and recharge. i think you take 4 lbs, but check the accumulator sticker. you may want to just change the accum at the same time.
SOURCE: raer a/c works but front blows hot air on my 99 tahoe
My 04 tahoe had to have the a/c door actuator replaced as well, same problem. 100.00 part from dealer if you want to do it yourself.
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